The present invention relates to surface plasmon devices and methods of making such devices and, more specifically, to surface plasmon guided wave devices whose optical frequency response is not limited by RC time constant.
One of the disadvantages of conventional electronic devices based on electron charge oscillation is that they cannot operate at terahertz frequencies or higher due to time constant limitations. In tunneling junction devices, for example, the speed of the device is limited by the fact that the outer conductive layers of the tunneling junctions act as parallel plate capacitors. Coupled with the internal and external resistance to current flow, the RC time constant constrains the speed of the tunneling junction device. Thus, only very small devices, which have correspondingly small capacitance, can be operated at optical frequencies of 1013–1015 Hz.
One way to circumvent the RC time constant limitation and get optical frequency response from these devices is to use traveling waves. In metal-insulator based structures, such as metal-insulator-metal tunnel junctions, surface plasmons provide such a solution. Surface plasmons are TM electromagnetic modes which propagate along the interface between two materials having dielectric constants of opposite signs, such as the interface between a metal and a dielectric material or the interface between a metal and a semiconductor. The use of surface plasmons as a means of transferring and manipulating electromagnetic energy in a device is advantageous because the electromagnetic energy is tightly confined when in the form of surface plasmons. Therefore, small, compact devices for controlling electromagnetic energy are enabled by taking advantage of the properties of surface plasmon propagation.
Research into devices based on surface plasmon transmission has focused on devices formed of semiconductor materials and on devices formed of metal-insulator combinations. Studies on semiconductor structures have centered mainly on resonant tunneling diode structures based on multiple quantum wells. For example, Korotkov et al. examine the theoretical possibility of lasing from double quantum well (or triple-barrier) heterostructures.1 Korotkov et al. describe the use of radiative, interwell transitions (i.e., inelastic tunneling between adjacent quantum wells) to achieve lasing. Korotkov et al. also suggest the minimization of absorption of terahertz radiation in doped gallium-arsenide by using periodically-spaced p+ and n+ doped injection layers.
Whereas the paper of Korotkov et al. is based on theory, Drexler et al. discuss experimental behavior of a semiconductor triple-barrier resonant tunneling diode (RTD) with a bowtie antenna.2 Drexler et al. observed evidence of stimulated absorption and emission of terahertz fields in the I-V curve of their experimental device, indicating photon-assisted tunneling (which they interpret as stimulated absorption of photons) in the RTD. Drexler et al. further suggest that the device may be adopted to design tunable quantum detectors and radiation sources in the terahertz radiation range.
An approach toward attaining gain in solid state plasmas in semiconductor structures is discussed by Kempa et al.3 The article of Kempa et al. discusses the possibility of attaining growing coherent plasma oscillations (i.e., plasma instability or stimulated emission of plasmons) using quantum well structures with the aim of fabricating a terahertz source based on the resulting gain. By interpreting the experimental and theoretical nonlinearity of the I–V curve of their semiconductor devices, Kempa et al. assert the presence of population inversion and predict the achievement of stimulated emission of plasmons using their semiconductor structure scheme if the structure is optimized.
Estimated gain in a triple barrier (i.e., double quantum well) RTD structure is discussed by Asada et al.4 By separating the photon absorption and stimulated emission peaks from the total current curve in the I-V characteristics of the device, Asada et al. estimate the net stimulated emission rate of the tunneling electrons as a function of bias voltage. Thus, Asada et al. calculate the estimated intersubband terahertz gain and predict that high gain is possible in the device by increasing the current density in the device as well as by using thin barriers.
A surface plasmon quantum cascade (QC) laser based on chirped semiconductor superlattices has been demonstrated by Tredicucci et al.5 The surface plasmon QC laser of Tredicucci operates at a wavelength of λ˜19 μm, and relies on guided surface plasmon modes at a metal-semiconductor interface to effectively confine the infrared wavelength light. The surface plasmon QC laser is operable up to a maximum temperature of 145K.
Semiconductor-based devices have several drawbacks. Due to their crystalline nature and the fact that deposited semiconductor layers must epitaxial, semiconductor-based devices are relatively expensive to fabricate and very difficult to integrate with other optoelectronic devices. In addition, semiconductor carrier concentration is limited to <1020 cm−3, which is about 1000 times lower than in metals. One consequence of the lower carrier concentration is that the plasma frequency is reduced from optical frequencies to millimeter wave frequencies, so that optical waves cannot be carried by semiconductor surface plasmons. Another consequence is that the device bandwidth is reduced due to transport resistance. Still another consequence for amplification and emission devices is that the increased resistance would limit the output intensity and increase heating.
An alternative to semiconductor-metal structures is the use of a metal-insulator interface to confine the surface plasmons. Metal-insulator layers are relatively inexpensive to manufacture in comparison to semiconductor-based systems. As the oxides of metals are generally used as the insulator materials, structures including metal-insulator interfaces are straightforward to fabricate compared to the epitaxial growth techniques for semiconductor layers. Due to the high carrier concentration in metals and the extremely short transit time of tunneling electrons, metal-insulator-metal (MIM) tunneling junctions are potentially useful in devices for optical frequency electronics.
Research on surface plasmons in MIM devices published in the 1970's and 1980's focused largely on developing an understanding the properties of point contact tunneling junctions, which were shown to detect infrared and millimeter wave radiation, and on light-emitting tunneling junctions, which give off a broad spectrum of spontaneously-emitted visible and infrared radiation.
An emitter based on the MIM diode structure is disclosed by Lambe et al. in U.S. Pat. No. 4,163,920.6 The emitter of Lambe includes a strip of metal, with an oxide formed thereon, and a second metal strip crossing the first metal strip and oxide, thus forming an MIM structure at the intersection of the two metal strips. The top surface of the second metal strip is roughened such that the second metal strip is rendered slightly porous. By applying a current across the two metal strips, Lambe et al. achieve light emission from the excitation of surface plasmons in the tunneling junction due to inelastic tunneling of electrons. In other words, the emitter of Lambe et al. is essentially a broadband emitter in the visible spectrum with the light being emitted from the roughened surface of the MIM junction.
The emitter as described in Lambe et al. has a number of drawbacks. One such drawback is the relatively inefficient spontaneous emission process of surface plasmons in single-insulator MIM diodes. For example, Volkov and Chuiko estimate the efficiency of this transition to be ˜10−4.7 To improve the spontaneous emission efficiency, Aleksanyan et al. investigated insulator structures with one or more quantum wells in which the quantized energy levels inside the potential wells limit the available tunneling states to those for inelastic tunneling in which a surface plasmon is generated.8 A problem with embodiments in the literature devices based on multiple quantum well structures is the non-uniformity of the barrier and quantum well layers. As another approach, Belenov et al. demonstrated enhanced light emission in evaporated MIMIMIM triple-barrier diodes over single-barrier diodes.9 However, Belenov et al. attribute the increased light intensity to increased scattering in the films with additional layers. It is submitted that traditional thin film deposition techniques, such as evaporation, are not well suited for growing very thin (<5 nm) uniform layers of amorphous materials due to the difficulty in controlling the uniformity of layers. Another drawback of the Lambe device is the inefficient output coupling of surface plasmon waves to optical radiation. In their device, and most others in the literature, surface roughness is used to transform the confined surface plasmon wave in the MIM junction into radiation. This approach has the disadvantages that the optical power must traverse the thickness of the top metal layer before radiating into free space, which is a lossy process. Furthermore, for some applications, surface roughening is a rather uncontrolled method to couple out the light as the light will tend to radiate equally into the hemisphere above the device.
Several researchers also theorized the possibility of stimulated emission of surface plasmons in MIM diodes. For example, Siu et al. discuss the possibility of stimulated electron tunneling in a point contact MIM diode structure.10 Siu et al. assert the existence of stimulated electron tunneling due to surface plasmon excitation in the point contact MIM structure as demonstrated in the negative differential resistance exhibited in theoretical calculations and in preliminary observations using high resistance, small area tungsten-on-gold point contact diodes. However, Siu et al. do not suggest practical applications for the phenomenon. As another example of theoretical research, Drury and Ishii discuss stimulated emission of surface plasmons in metal-insulator-metal diodes and suggest their use as coherent optical sources.11, 12 Also, Aleksanyan et al. and Belenov et al. theorized on the amplification of surface plasmons in MIM junctions by stimulated emission.8, 13, 14, 15 Aleksanyan et al. and Belenov et al. analyzed double- and triple-barrier structures, predicting near unity quantum efficiency in structures with two quantum wells (triple barrier) such that the plasmon-producing transition was due to the inelastic tunneling between quantum levels in these two wells. In one report, Belenov et al. discuss experimental results from a Au—Al2O3—In2O3—Al2O3—In2O3—Al2O—Ag diode, showing a slight peak in the luminescence output corresponding to a peak in the current-voltage curve (a region of negative differential resistance).15 They do not, however, show definitive amplification of surface plasmons. Xie also considers stimulated emission and absorption of surface plasmons in MIM transition metal type like junctions.16 While Xie shows that stimulated emission of surface plasmons is theoretically possible in MIM devices, the material selection in the MIM diode as discussed in the article is very limited. Namely, the collector of the MIM diode in the article of Xie is required to be a transition metal, the insulator layer must be very thin (around 10 Å), and the stimulated emission only occurs at wavelengths longer than λ˜10 μm. Further, Xie restricts the outer metal layers of the MIM to single crystal metals. However, Xie does demonstrate the possibility of obtaining large gain from such devices and suggests that such a structure might be useful as an amplifier or emitter in the infrared region, although Xie does not discuss the possibility of gain in the visible or other spectra. Xie further suggests that the electron tunneling can be used as a pumping mechanism in plasmon devices with micro-dimensions, although he does not propose specific ways in which such devices might be implemented.
None of the above cited references discuss specific device structures, for example, for optical amplifiers or lasers, such as the configuration of input and output optical couplers or of resonant cavity design, but discuss only the electronic structure of the tunneling junction. Furthermore, none of the above references suggest methods for producing the necessary thin, uniform barrier and well layers for these devices.
Several researchers have proposed optical modulators based on the coupling of light with surface plasmon waves. In nearly all cases, reflected or waveguided light is attenuated by coupling into the surface plasmon mode, and this coupling is controlled by changing the index of refraction of a dielectric layer adjacent to the metal film. Thus, a fairly strong electro-optic effect is required (Δn˜10−3–10−2) to operate the optical modulator. For example, Schildkraut proposed a reflection modulator in which light internally reflected within a prism can couple to a thin metal film on the backside of the prism by means of an electro-optic material adjacent to the metal film.17 With an electro-optic material of second order susceptibility (e.g., 2×10−7 esu), Schildkraut reports that a 100 volt signal modulates the optical reflectance from 0.00 to 0.84. As another example, Jannson et al. disclose a high-speed light modulator in U.S. Pat. No. 5,067,788, which includes a buffer and an electro-optic material sandwiched between two metal electrode layers, positioned adjacent to and separated from an optical waveguide (such as an optical fiber).18 A surface plasmon wave is generated on the metal electrode adjacent to the optical waveguide by evanescent wave coupling. Applying a voltage across the electro-optic material changes the index of refraction of the material and thus the strength of the surface plasmon coupling. The optical intensity within the optical waveguide is therefore modulated in the device of Jannson.
As another example, in U.S. Pat. No. 6,034,809 Anemogiannis discloses an optical plasmon wave attenuator and modulator structures in which the output optical power is attenuated or modulated by controlling the amount of coupling between a guided optical signal and a separately generated surface plasmon wave.19 Anemogiannis further considers the phase matching conditions for the coupling wave vectors to achieve efficient attenuation and modulation. Similarly, Janunts and Nerkararyan also proposed a modulator in which light is incident on a metal film through a prism coupled into a surface plasmon wave in the metal and which, in turn, radiates into an optical mode in a dielectric waveguide.20 Coupling of the light into the waveguide is controlled by an electro-optic material on top of the metal film and dielectric waveguide. Again, large changes in index of refraction, Δn>0.001, are required to produce modulation.
All of the aforementioned modulators require the use of a suitable electro-optic material, such as a non-centrosymmetric crystal or nonlinear polymer. This requirement adds cost to device fabrication and limits device integration with other optoelectronic devices. In addition, these specialized materials generally require large voltages (10–100 V) to attain the necessary refractive index change for modulation.
The present invention provides surface plasmon devices which is intended to reduce or eliminate the foregoing problems in a highly advantageous and heretofore unseen way and which provides still further advantages.